Title.
Signal can then claim it has no control over what user-made Add-ons are installed and used for. They just support the framework, which is not illegal, just like media players support playing illegal content but don’t create such content.
For people out of the loop: EU soon likely enacts Chat Control 2.0 which will force chat services to hand over keys so governments can read all your personal messages.
And couldn’t bad actors just host some kind of network of Matrix servers that aren’t accessible on the public web and the whole thing is moot anyways?